Run GroupByV2 queries in compatibility mode on Historicals

[niketh]

Run GroupByV2 queries in compatibility mode on Historicals. If the broker doesn't send "groupByStrategy" in the context

[niketh]

@gianm Can you review

[gianm]

I don't think this will help: the compatibility mode in this patch just changes how results are compared when merging, which wouldn't solve the issue on 0.9.1.1 brokers. They'll still get null timestamps and still not be able to handle them. If we want 0.9.1.1 brokers to work with 0.10.0 historicals then the historicals behavior needs to change.

Looking into it a little more, we kind of already have a natural 'compatibility flag' the historicals can use to detect what behavior they should do: CTX_KEY_OUTERMOST, which is set to "false" by new brokers but is null from older brokers. Most of the code to make this work is already there, the only missing piece was that GROUP_BY_MERGE_KEY was being used by both engines to disable mergeResults, but it really should only be used by the v1 engine. v2 should always merge results.

I raised #4023 to do this, @niketh could you take a look please?